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Strike Termination

Multiple strike termination devices must be positioned in specific locations at the top of a structure where lightning can be expected to terminate. These lightning rods create a "zone of protection" for the building as determined by a model that simulates how lightning interacts with ground-based objects known as the rolling sphere analysis

Roof-top Elements of Lightning Protection Systems

Lightning protection standards mandate that strike termination devices are positioned at the structure’s high points, spaced at regular intervals not exceeding 20 to 25 feet along ridges and roof perimeters, and at corners of roofs. On large buildings, the field of the roof must also have strike termination devices.

In addition, anything projecting above the roof – such as parapets, HVAC units, hatches, and solar collectors – will also require strike termination devices unless they are in the zone of protection created by a higher part of the structure.

There are two types of strike termination devices:

1. Air terminals (informally known as “lightning rods")

2. Roof-top metal fabrications complying with specific criteria

At Roof Level

Air terminals consist of a mounting base and a vertical rod. Bases should be mounted using techniques acceptable to the roofing or substrate manufacturer. Rods from a lightning rod supplier must extend a minimum of 10 inches above the object they protect. Air terminals should be located as close as practical to the roof edges and the ends of ridges however they may not be set-back more than 24 inches. 

Air terminals with rounded tips have been shown to be more effective than the pointed air terminals that were commonly used in the past. The blunt-tipped rods are also safer for personnel working on the roof.

Creative Use of Rooftop Metal Fabrications

Lightning protection standards allow roof-top metal building elements to be used in lieu of air terminals if they are permanent parts of the building, electrically continuous with the lightning protection system, and made of metal at least 3/16 inch (0.064 inch for handrails) thick. Examples of such metal fabrications include metal roof ladders, parapet caps, signs, sculptures, decorative items, canopies, and more.

Getting double-duty out of these metal building features can reduce the cost of construction. It also provides aesthetic and creative options, especially for occupied roofs and other locations where people may be in close proximity to the lightning protection system.

Additional examples of creative incorporation of metal building elements into the lightning protection system can be found here and also in the article "Lightning Protection Systems on Rooftop Terraces."

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